Ophthalmic refractors are used to measure ocular refractive error (sphere and cylinder). There are various forms of refractor suitable for measuring ocular refractive error; one form uses a device originally designed to measure ocular aberrations, and is called a Hartmann-Shack aberrometer.
A Hartmann-Shack aberrometer typically measures ocular wavefront aberrations by delivering a parallel beam of near-infrared radiation to the eye. A Hartmann-Shack wavefront sensor is positioned to receive the wavefront leaving the eye. This wavefront sensor includes a relay system, lenslet array and image sensor, which may be a charge-coupled device (CCD), a CMOS sensor, or a video camera. The position of the received infrared radiation on the image sensor the extent of wavefront aberration.
The relay system of a Hartmann-Shack wavefront sensor is an afocal (telescopic) system of two lenses which images a reference plane coincident with the pupil plane onto the lenslet array, typically with a unit magnification (1×). Using this approach, the wavefront curvature at the lenslet array is equal to the wavefront curvature in the pupil plane. The lenslet array is conjugate to the pupil plane and the image sensor is conjugate to the retinal plane.
A Hartmann-Shack wavefront sensor of the type described above has a relay system with a total length of four times the focal length. This long optical path can limit or constrain the uses to which the aberrometer can be put, or at least reduce the convenience of use.